Legislation in the UK requires that a tire should have at least 1.6 mm of tread over 75% of it's road surface contact area for a car tire. Different limits apply for the other types of vehicles. Hitherto, the estimation of tread depth, and more particularly, whether this depth is maintained across the required width of the road contact area, has been performed mainly by manual inspection.
WO 96/10727 discloses a tire scanning apparatus in which a light source and a photodetector are mounted in a carriage which is driven back and forth along a scan path by a motor and drive belt. The use of a mechanical scanning system of this type may lead to an unacceptable failure rate of the system.